Bottle conditioning of beer: Strategies to improve yeast refermentation performance

Technical Session 14: Yeast II Session
Tinne Dekoninck, Catholic University of Leuven, Heverlee, Belgium
Co-author(s): Filip Delvaux and Freddy Delvaux, Catholic University of Leuven, Heverlee, Belgium

ABSTRACT: Beer refermentation, i.e., bottle conditioning, is a frequently used technique among breweries in Belgium, the United Kingdom, and the United States. To achieve a secondary fermentation in the bottle, mature beer is inoculated with yeast and fermentable extract, whereupon it is refermented in preferably less than two weeks. Bottle conditioning results in fully saturated beer with an enriched flavor perception and prolonged flavor stability. Since export and consumption of bottle conditioned beers still increases, it is of major economic importance that constant product quality can be assured. Although beer refermentation seems uncomplicated, the process faces important pitfalls presumably because of yeast stress. Indeed, beer is far from an excellent fermentation medium since it differs from an ideal wort medium in its alcohol and carbon dioxide content and low nutrient availability. To improve the process of bottle conditioning, several perspectives can be considered. In a first experiment, the refermentability of several Belgian beers (both lager and ale types) was investigated to reveal the impact of beer related parameters on refermentation. A striking finding was a strong influence of initial beer alcohol levels on refermentation performance, especially when a less ethanol tolerant yeast strain was used. To improve the refermentation performance of different yeast strains, a promising strategy could, therefore, be the adaptation of yeast to alcohol, prior to beer inoculation. In a second experiment, yeast was propagated both in a dynamic and static way, with variable extract and alcohol levels. Throughout propagation and refermentation, important yeast physiological parameters were monitored, such as viability, glycogen and trehalose content, fatty acid and ergosterol levels, as well as the expression of (stress related) genes. These analyses revealed physiological differences between statically and dynamically propagated yeast, as well as between alcohol conditioned and reference yeast populations. Our findings indicate that the use of appropriate conditioning of yeast provides promising opportunities to increase yeast refermentation performance during bottle conditioning of beer.

Tinne Dekoninck graduated in 2008 as a bio-engineer in chemistry (food technology) from the Catholic University of Leuven. For her M.S. thesis, she joined the Centre for Malting and Brewing Science to study the feasibility of high cell-density brewery fermentations. After graduation, she obtained a grant from the Institute for the Promotion of Innovation Through Science and Technology in Flanders (IWT) and started a Ph.D. program at the Centre for Malting and Brewing Science, under the supervision of Freddy Delvaux. Her research focuses on the impact of yeast physiology on bottle conditioning of beer.